106 Non-restrictive Clauses (English)

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Understanding Restrictive Versus Non-Restrictive Clauses

Restrictive clauses and non-restrictive clauses are generally known by the 'that versus which' question we ask ourselves when determining when to use a comma before placing a relative pronoun in a sentence. As a general rule of thumb, a comma usually precedes 'which' and no comma before the word 'that.'
Sometimes though, it helps to understand the finer points of the grammar rules governing this confusing comma usage. 'That' is the word corresponding to a restrictive clause, and 'which' is the word corresponding to a non-restrictive clause. When 'that' is used in a sentence, the restrictive clause that follows is also known as a subordinate clause or a dependent clause.
Perhaps it helps to remember that commas are used to separate independent things, such as items in a grocery list or two sentences joined by the conjunction 'and.' Therefore, the restrictive clause ('that') does not get a comma because there is a dependency and the two objects being discussed are not independent of each other. In other words, a restrictive clause restricts, or limits, the subject of the sentence to only include the subcategory, which is provided after the relative pronoun.
For example, only cats that are orange" is restrictive because there are many cats, but the sentence is about to discuss only the ones that are orange. In the sentence: "the cats, which are orange," the relative pronoun 'which'is used because there are only orange cats, so no further definition, or restriction, is needed to identify the subject of what is about to be discussed.

For more information see Restrictive Clauses.